This invention pertains to the art for testing specimens for fastness and deterioration under light, and more particularly, to such an apparatus using discharge lamps as light sources.
This invention is particularly applicable to testing of specimens using fluorescent ultraviolet lamps to simulate the deterioration caused by sunlight and will be described with particular reference thereto. However, it will be appreciated that the application has broader applications and may be advantageously employed in apparatuses using xenon lamps and in connection with many other accelerated weather testing concepts and uses.
A conventional testing apparatus using discharge lamps as shown in FIG. 1 has eight ultraviolet fluorescent lamps 10 provided in a test chamber 12 and arranged into symmetric downwardly divergent rows when viewed in cross-section. Specimens 14 to be tested are attached to two opposite specimen supporting walls of the housing of the test apparatus so as to face inwardly toward the fluorescent lamps and receive the light irradiance therefrom. In the machine shown, there are two specimens, an upper and lower one. However, there may be only a single specimen or more than two. The rear surfaces of the specimens 14 are exposed to the atmospheric air outside the machine. Outside air is heated and blown into the interior of the chamber 12 to regulate the temperature in the chamber 12. Water in moisture supply tank 16 is made hot and evaporated to supply moisture into the chamber 12.
In the above-described testing machine, one example of the machine's operation includes applying ultraviolet rays to the specimens 14 at a temperature of 60.degree. C. for 16 hours and the fluorescent lamps 10 are turned off and the interior of the chamber 12 is kept at 50.degree. C. for eight hours. These two steps, which constitute one cycle of a deterioration testing operation, are repeated continuously. While the fluorescent lamps are off, the humidity in the chamber 12 is high, and the rear surfaces of the specimens are exposed to the outside air at a low temperature. Accordingly, the surface of the specimens are wetted due to condensation. Thus, the wetting of the specimens, the applying of ultraviolet rays, and the drying are repeated, which speeds the deterioration of the specimens. It is to be appreciated that the above description is just one type of cycle for which machines of this nature can be used.
Problems, however, exist with the apparatus shown in FIG. 1. Initially, there is no provision for sensing the output of the fluorescent lamps 10, in order to track their rate of degradation. A normal procedure for attempting to provide a uniform output from the lamps, in such a device, is to rotate the positions of the lamps at predetermined time intervals in a predetermined sequence. Testing of the lamps to detect actual output is not provided, rather, assumptions are made as to the likely output, and the rotation sequence is made in consideration of the assumptions.
An additional drawback of this type of device is that the discharge lamps 10 which are located on one side of the chamber 12 transmit light beams to the opposite side of the chamber. For example, the row of lamps on the left side of the chamber in FIG. 1 are intended to produce irradiance for the specimen 14 also on the left side of the chamber. However, these lamps also produce beams in an undesirable fashion on the specimens 14 On the right hand side of the chamber 12. These undesirable beams tend to concentrate towards the middle of the specimen supporting wall. Therefore, a common problem is having the specimens which are located nearest the middle of the testing apparatus receiving higher doses of irradiance than those specimens arranged toward the top or bottom of the apparatus. This decreases the uniformity with which irradiance is transferred to the specimens.
Various attempts have been made to improve on the above-noted drawbacks of the conventional testing apparatus shown in FIG. 1. Among these is an apparatus from Atlas Electric Devices Company, called Atlas Ci35 FADE-OMETER.RTM.; an apparatus from Heraeus called XENOTEST.RTM. 1200 CPS; U.S. Patent to Suga, U.S. Pat. No. 4,544,995 issued Oct. 1, 1985; and U.S. Patent to Kockott, et al., U.S. Pat. No. 4,544,995 issued Apr. 27, 1971.
The Atlas device is arranged for use with a xenon arc lamp and includes a closed loop irradiance monitor as its primary light control system. The monitor, using a light pipe, interference filter and photosensitive diode feeding into solid state electronics, maintains predetermined irradiance levels and totalizes the energy received by the samples through an integrator. This device is also equipped with manual irradiance controls for use when periodically calibrating the system.
The apparatus from Heraeus is also directed for use with xenon arc lamps. This device employs three light detectors to detect the output of three individual xenon arc lamps.
A conventional apparatus including elements of these two above-discussed devices is shown in FIG. 2. In this Figure, discharge lamps 30 which can be of a xenon type, are vertically disposed. A filter 32 surrounding the discharge lamps 30 is provided to allow only desired wavelengths of light to pass. Sensors 34 are provided to sense the output of the vertically positioned discharge lamps 30, and a rotating specimen holding rack is positioned to encircle the discharge lamps 30. Each of the detectors 34 are provided to detect the irradiance produced from a respective discharge lamp 30 over time. The rotating specimen holding rack 36 rotates the specimens located in the specimen holding rack 36. The sensors 34 are provided to track the output of the discharge lamps 30, and the rotating specimen holding rack 36 attempts to provide each of the specimens with an average overall equal amount of irradiance. Inner walls 38 are used to direct reflective light of the discharge lamps 30 outward to the specimens.
Another device, employing ultraviolet lamps in an arrangement similar to FIG. 1, is known to include a single sensor. However, in such an arrangement it is necessary to match the characteristics of the lamps prior to placing them in such a device. This is required since the sensor will only sense the lamps closest to its location. Thus, the sensor will assume the lamps placed distant from it are operating the same as the lamps it actually senses.
The Suga patent attempted to improve on the prior art device shown in FIG. 1 by adjusting the alignment of the row of discharge lamps 10 of FIG. 1 into a non-symmetric arrangement. This arrangement is shown in FIG. 3. As noted in this Figure, the discharge lamps 10 are not disposed immediately below each other. Rather, they are in a specifically positioned arrangement. This was done in Suga in an attempt to provide irradiance to the samples 14 with a more uniform distribution. The numeral designations shown in FIG. 3 are millimeter (mm) measurements.
The Kockott, et al. patent is directed to a device using an elongated source of irradiation inside a cylindrical carrier surface. Kockott, et al. discloses three approaches to provide a uniform distribution of irradiance to the samples. First, mirrors 14 are arranged to reflect usable light; second, a light source 20 is designed to increase light intensity at its ends; and, third, collimating discs 24 are used to inhibit divergence of the radiation emitted from the source.
While the above-discussed references provide some improvements upon the conventional apparatuses shown in FIG. 1 and FIG. 2, drawbacks still exist.
With particular attention to the Atlas and Heraeus devices, it is noted that both use a rotating specimen rack arrangement. This rack is necessary for a very basic reason. The Atlas device includes a monitoring system which monitors the overall output of the xenon arc lamp in order to attempt to maintain a predetermined total irradiance output level over time for the entire system. The Heraeus device uses three sensors to control the three different lamp's output over time. These sensor arrangements are used to produce an irradiance which is constant over time. However, neither of these devices use a sensing arrangement to make irradiance constant over space.
Both of the devices use a rotating specimen rack in an attempt to achieve spatial uniformity. Therefore, spatial uniformity which is achieved, is accomplished by having the specimens in the rotating rack revolve around the lamps, so the effective light dosage received by each specimen is an average of the different irradiances at each point on the circumference of the sample plane. Though rotating the rack increases uniformity, it also increases the complexity of the device by requiring a motor and associated rotation mechanisms.
Thus, even though these devices include irradiance sensing capabilities, they implement these capabilities only for a consistent output over time, not space. As can be seen in FIG. 2, due to the geometry of the devices, there is a different irradiance at every point around the circumference of the sample plane. Therefore, areas which are located in front of a discharge lamp 30 will have a high irradiance area H while samples which are at a position distant from a discharge lamp 30 will receive lower L irradiance. Rotation of the rack attempts to produce an overall average uniformity of irradiance impinging upon samples.
The known ultraviolet system using a single sensor includes the drawback of needing to match the lamps being used in the system. This requires extensive testing of the lamps prior to use. A further drawback is that in such a system, when a lamp located distant from the sensor location burns out or degrades, the decrease in its output will not be sensed. This is true as only the nearest lamps are actually sensed and an assumption is made that the remaining lamps are functioning in a similar manner.
The Suga patent attempts to increase the uniformity of light impinging upon specimens by moving the center two lamps away from the samples to increase uniformity of light to the samples from top to bottom. A drawback of such an arrangement is that it is not possible to easily retrofit existing weathering devices to gain whatever improvement there may be from the Suga arrangement.
A drawback to the Kockott, et al. patent is that it is directed to single lamp systems. Another drawback to Kockott, et al. is that it increases the complexity and cost of the apparatus.
A further drawback associated with the conventional testing apparatuses as discussed above is their calibration. These devices require manual manipulations by an operator which in turn means the operator is required to make decisions which are critical to proper calibration. Since the operator is responsible for making decisions while manually re-calibrating the apparatus, the accuracy of the calibration will be dependent upon the skill of the operator. Additionally, since the calibration is accomplished manually, extended down time occurs during such calibration and there exists a substantial possibility of inaccuracies due to operator error.
The subject invention contemplates a new and improved accelerated weathering apparatus that overcomes all of the above referenced problems and others and provides an easily assembled, reliable testing structure.